During flight, undesirable electrical energy can build up in aircraft fuel tanks. This can be in the form of static electricity or electrical energies induced in the tank due to a lightning strike. If this energy is allowed to build to a great enough level then there exists a significant risk of an ignition event occurring in which the aircraft fuel, in particular fuel vapour, in the fuel tanks will ignite. It is thus desirable to limit the amount of electrical energy that is able to exist within the tank.
Fuel tanks of the prior art made of metal typically offer a certain amount of protection from lightning by virtue of the tank acting as a Faraday cage. However, composite fuel tanks do not afford such inherent protection. Such a disadvantage of composite fuel tanks can, to a limited extent, be mitigated by means of having a metallic mesh embedded within the composite structure. However, such measures provide little protection in the case of larger composite fuel tanks, where in-tank conductive components increase the risk of an ignition event resulting from lightning. That risk can be reduced by insulating the in-tank conductive components, but that can increase the risk of static charge build-up becoming hazardous.
Fuel tank safety has over recent years been recognised as being particularly important in maintaining aircraft safety. Various regulations have been laid down by the relevant official bodies governing aircraft design. For example, the FAA (the US Federal Aviation Administration) has set various criteria for fuel tank design in aircraft in Special Federal Aviation Regulation (SFAR) No. 88. It is of course highly desirable for any modifications or improvements of fuel tank design to be compatible with (SFAR) No. 88 and/or any similar regulations in other countries or which supersede (SFAR) No. 88.
Due to the potentially catastrophic consequences, should an ignition event occur in an aircraft fuel tank, whatever means used for reducing the level of the risk on the level of electrical energy within the tank should ideally be highly reliable. It is desirable for example that these means should be of a safety-critical nature. For example, the means should preferably be in the form of a fail-safe system.
One safety system of a fuel tank of the prior art simply comprises one or more discharge paths from the fuel tank to the aircraft ground, in the form of grounding straps mechanically bonded to the tank and to the aircraft structure. Such straps are required to be routinely manually visually inspected to ensure that the local bonding points maintain good conductive contact. Such manual checks can only be performed when the aircraft is stationary and grounded and are not especially reliable.
The present invention seeks to provide a fuel tank with improved safety features and/or a fuel tank, which mitigates one or more of the problems or disadvantageous features of the prior art mentioned above and/or meets one or more of the above-mentioned criteria that it would be desirable to satisfy.